Simplify services using termux-services

[brad]

This commit refactors the service management logic to use the official termux-services (runit) package.

Key changes:

• Updated setup.sh to install termux-services and configure a boot script to start runsvdir.
• Implemented a setup_service helper in services/utils.sh to standardize runit service creation.
• Refactored all service scripts (SSH, Syncthing, Transmission, Mosquitto, Navidrome, Nextcloud, Tailscale) to use sv-enable and setup_service.
• Replaced manual backgrounding logic and per-service boot scripts with runit process management.
• Updated shortcuts to use sv down for stopping services.
• Updated README.md to document the move to termux-services.

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PR created automatically by Jules for task 2883056356091608676 started by @brad

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